How many of the following statements concerning resonance structures are correct?

Question

I. The concept of resonance is used because the lewis structure model is incomplete in describing bonding in a molecule.

II. For a species having 3 resonance structures, it is best to think of the species to exist as each of these structures one-third of the time.

III. All charged molcules have resonance structures.

IV. The Octet rule must not be violated when drawing lewis structures.

Answer 1

The only choice I like is the first one. The root of the problem is that quantum mechanics allows a bonding pair to be in a superposition of two (or three) states. In simpler language, a bonding pair can be in two (or three) different places at the same time. Lewis diagrams cannot really account for this quantum effect. Hence the kludge of resonance structures.

II is not good because X-ray analysis shows that there is just one bond length in a molecule like SO₂ not two.

A counter example to III is NH₄⁺

As for IV, free radicals like NO violate the octet rule, but nevertheless people do draw Lewis diagrams for them.